Genetic relationships of calving difficulty with birth measurements and carcass traits in Japanese Black cattle.

The objectives of this study were to compare the genetic parameters for calving difficulty (CD), which were treated as both a calf trait (CD_calf) and as a dam trait (CD_dam), and to clarify genetic relationships of these CDs with body size traits of calves at birth and carcass traits. In total, the CD records and calf body measurements of 2,258 Japanese Black cattle heifers were used in this study, in addition to the carcass records of 4,300 feedlot steers and heifers. Direct heritability of CD_calf (0.44) was higher than maternal heritability of CD_calf (0.30), as well as CD_dam heritability (0.25). Direct genetic correlations between CD_calf and calf body size were moderate to strongly positive (0.64 to 0.81). The correlations between EBVs of CDs and carcass weight were also positive (0.30 to 0.64). These positive relationships showed that genetically improving CD (reducing dystocia) could produce smaller calves and carcasses. In contrast, the correlations between CDs and beef marbling score were weak, suggesting that improving CD would not influence meat quality traits. Fitting an animal model to CD_calf could be more preferred to fitting the model to CD_dam, because the former could separate the genetic effects of dams and calves.